Abstract

Crosswell tomography of a sedimentary foundation at an iron foundry was affected by very high background noise; nevertheless, high-resolution velocity images were obtained between wells separated by long distances (120 to 250 m). A piezoelectric source in a water-filled well used long sequences (4095 cycles) of pseudorandom binary codes at high carrier frequencies (1 to 10 kHz). A 24-channel hydrophone array in another well received the signal. Beamforming of common-source data selected the directions and arrival times of multiple raypaths and tube waves and further enhanced the signal-to-noise ratio. Inversion of first-arrival times by damped least squares imaged the compressional wave velocities. Assuming the normal consolidation condition, the porosity and shear strength images are predicted from the compressional wave velocity image. The direct measurements of porosity and shear strength conducted on the cores and boreholes were used to verify the tomographic predictions. The slight differences in the compressional wave velocity images obtained using different carrier frequencies can be used to determine the permeability image of sediments based on the Biot theory.

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